Utilizing the capabilities of the National Science Foundation’s Very Large Array (VLA), astronomers have identified a substantial influx of gas in proximity to HW2. This colossal protostar, situated 2,283 light-years away within the star-forming expanse of Cepheus A, is facilitated in its accelerated development by this immense gaseous current.
Ammonia gas infalling into an accretion disk feeding the protostar HW2. Image credit: NSF / AUI / NSF / NRAO / B. Saxton.
The considerable accumulation of interstellar gas required for the genesis of a massive star, potentially dozens of times the mass of our Sun, spans vast celestial territories on the order of a parsec, equivalent to 3.26 light-years.
However, it is exclusively within circum-stellar zones approximately a few hundred astronomical units (AU) in extent that gas ultimately coalesces and is drawn onto a nascent protostar, which exhibits a diameter of merely about one million kilometers.
Characterizing the dynamics of gas streams as they flow into the inner few hundred AU of a very young star has presented a persistent observational hurdle, particularly for the most massive stars, which are located at significantly greater cosmic distances than stars analogous to our Sun.
“Our investigations furnish direct empirical validation that the formation of massive stars can proceed via disk-mediated accretion, achieving masses up to tens of solar masses,” stated Dr. Alberto Sanna, an astrophysicist affiliated with INAF and the Max-Planck-Institut für Radioastronomie.
“The VLA’s exceptional radio sensitivity empowered us to resolve structural details on scales as small as approximately 100 AU, thereby granting unparalleled insights into this formative phenomenon.”
Cepheus A stands as the second closest known stellar nursery where massive young stars with masses of ten solar masses or greater originate, rendering it an optimal locale for the detailed examination of these intricate processes.
Dr. Sanna and his research cohort employed ammonia, a molecular compound frequently encountered in interstellar gaseous nebulae and widely utilized in terrestrial industrial applications, as a diagnostic tool to delineate the gas motion surrounding the star.
The VLA observations revealed a dense annulus of heated ammonia gas extending across radial distances from 200 to 700 AU around HW2.
This structural configuration was identified as an integral component of an accretion disk, a cardinal element within established theories of stellar formation.
The investigating astronomers ascertained that the gas within this disk undergoes both inward collapse and rotational motion around the nascent star.
Conspicuously, the rate at which material accrues onto HW2 was quantified at two-thousandths of a solar mass per annum—representing one of the highest accretion rates ever documented for a forming massive star.
These discoveries corroborate the capacity of accretion disks to sustain such extreme rates of mass transfer, even when the central stellar body has already attained a mass equivalent to 16 times that of our Sun.
Furthermore, the research team juxtaposed their observational data with cutting-edge computational simulations of massive star genesis.
“The outcomes exhibited a strong congruence with theoretical projections, demonstrating that the ammonia gas in the vicinity of HW2 is collapsing at velocities approaching free-fall while simultaneously rotating at velocities below Keplerian—a delicate equilibrium governed by gravitational forces and centrifugal effects,” remarked Professor André Oliva, an astronomer associated with the Université de Genève and the Space Research Center (CINESPA) at the University of Costa Rica.
Intriguingly, the scientific endeavors brought to light disproportionalities within the disk’s architecture and its inherent turbulence, suggesting that external gaseous filaments, termed streamers, might be contributing fresh material to specific sectors of the disk.
Such streamers have been previously observed in other stellar nurseries and are posited to play a vital role in the replenishment of accretion disks surrounding massive stars.
This groundbreaking revelation effectively dispels decades of scientific discourse concerning whether HW2, and protostars of similar nature, can indeed establish accretion disks capable of supporting their rapid developmental trajectories.
It also lends considerable weight to the prevailing hypothesis that analogous physical mechanisms underpin stellar formation across a broad spectrum of stellar masses.
“This research not only enhances our comprehension of the mechanisms driving massive star formation but also holds significant implications for broader inquiries into galactic evolution and the chemical enrichment of the cosmos,” the authors articulated.
“Massive stars function as indispensable cosmic engines, propelling winds and orchestrating supernovae that disseminate heavy elements throughout galaxies.”
Their publication is scheduled for inclusion in the esteemed journal Astronomy & Astrophysics.
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A. Sanna et al. 2025. Gas infall via accretion disk feeding Cepheus A HW2. A&A, in press; doi: 10.1051/0004-6361/202450330
